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©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Machinery Fault Diagnosis A basic guide to understanding vibration analysis for machinery diagnosis.
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©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com ©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
This is a basic guide to understand vibration analysis for machinery
diagnosis. In practice, many variables must be taken into account. PRUFTECHNIK Condition Monitoring and/or LUDECA are not responsible for any incorrect assumptions based on this information.
© Copyright 2011 by PRÜFTECHNIK AG. ISO 9001:2008 certified. No copying or reproduction of this information, in any form whatsoever, may be undertaken without express written permission of PRÜFTECHNIK AG or LUDECA Inc.
Preface
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©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Unbalance
Unbalance is the condition when the geometric centerline of a rotation axis doesn’t coincide with the mass centerline.
Funbalance = m d ω2
m
ω
MP MP
1X
A pure unbalance will generate a signal at the rotation speed and predominantly in the radial direction.
Radial
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©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Static Unbalance
S
U →
m
U →
Static unbalance is caused by an unbalance mass out of the gravity centerline.
The static unbalance is seen when the machine is not in operation, the rotor will turn so the unbalance mass is at the lowest position.
The static unbalance produces a vibration signal at 1X, radial predominant, and in phase signals at both ends of the rotor.
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Pure Couple Unbalance
m
S
UΙ →
-m
b
UΙΙ = -UΙ → →
Pure couple unbalance is caused by two identical unbalance masses located at 180° in the transverse area of the shaft.
Pure couple unbalance may be statically balanced.
When rotating pure couple unbalance produces a vibration signal at 1X, radial predominant and in opposite phase signals in both ends of the shaft.
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Dynamic Unbalance
m
S -m
UΙΙ →
UΙ →
Dynamic unbalance is static and couple unbalance at the same time.
In practice, dynamic unbalance is the most common form of unbalance found.
When rotating the dynamic unbalance produces a vibration signal at 1X, radial predominant and the phase will depend on the mass distribution along the axis.
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©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Documentation of balancing
Frequency spectra before/after balancing
and balancing diagram.
.. after balancing
before ..
Balancing diagram
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©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Overhung Rotors
A special case of dynamic unbalance can be found in overhung rotors.
The unbalance creates a bending moment on the shaft.
Dynamic unbalance in overhung rotors causes high 1X levels in radial and axial direction due to bending of the shaft. The axial bearing signals in phase may confirm this unbalance.
1X
Radial
Axial
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Unbalance location
The relative levels of 1X vibration are dependant upon the location of the unbalance mass.
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Misalignment
Misalignment is the condition when the geometric centerline of two coupled shafts are not co-linear along the rotation axis of both shafts at operating condition.
1X 2X MP MP
A 1X and 2X vibration signal predominant in the axial direction is generally the indicator of a misalignment between two coupled shafts.
Axial
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Angular Misalignment
Angular misalignment is seen when the shaft centerlines coincide at one point along the projected axis of both shafts.
The spectrum shows high axial vibration at 1X plus some 2X and 3X with 180° phase difference across the coupling in the axial direction.
These signals may be also visible in the radial direction at a lower amplitude and in phase.
1X 2X 3X
Axial
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Parallel Misalignment
Parallel misalignment is produced when the centerlines are parallel but offset .
The spectrum shows high radial vibration at 2X and a lower 1X with 180° phase difference across the coupling in the radial direction.
These signals may be also visible in the axial direction in a lower amplitude and 180° phase difference across the coupling in the axial direction.
1X 2X 3X
Radial
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©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Misalignment Diagnosis Tips
In practice, alignment measurements will show a combination of parallel and angular misalignment.
Diagnosis may show both a 2X and an increased 1X signal in the axial and radial readings.
The misalignment symptoms vary depending on the machine and the misalignment conditions.
The misalignment assumptions can be often distinguished from unbalance by:
• Different speeds testing
• Uncoupled motor testing
Temperature effects caused by thermal growth should also be taken into account when assuming misalignment is the cause of increased vibration.
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©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Alignment Tolerance Table
The suggested alignment tolerances shown above are general values based upon experience and should not be exceeded. They are to be used only if existing in-house standards or the manufacturer of the machine or coupling prescribe no other values.
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Shaft Bending
A shaft bending is produced either by an axial asymmetry of the shaft or by external forces on the shaft producing the deformation.
A bent shaft causes axial opposed forces on the bearings identified in the vibration spectrum as 1X in the axial vibration.
2X and radial readings can also be visible.
1X 2X
Axial
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Rotating Looseness
Rotating looseness is caused by an excessive clearance between the rotor and the bearing
Rotation frequency 1X and harmonics
Radial
Rotation frequency 1X Harmonics and sub Harmonics.
Radial
Rolling element bearing:
Journal bearing:
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Structural Looseness
Structural looseness occurs when the machine is not correctly supported by, or well fastened to its base.
MP
MP
1X
Radial
• Poor mounting
• Poor or cracked base
• Poor base support
• Warped base
MP MP
Structural looseness may increase vibration amplitudes in any measurement direction. Increases in any vibration amplitudes may indicate structural looseness.
Measurements should be made on the bolts, feet and bases in order to see a change in the amplitude and phase. A change in amplitude and 180° phase difference will confirm this problem.
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Resonance
Resonance is a condition caused when a forcing frequency coincides (or is close) to the natural frequency of the machine’s structure. The result will be a high vibration.
1st form of natural flexure
2nd form of natural flexure 3rd form of natural flexure
t
v
tx
t
v
tx t
v
tx
f f1st nat, flexure f2nd nat, flexure f3rd nat, flexure ⇒ no harmonic relationship
t
Shaft 1st, 2nd and 3rd critical speeds cause a resonance state when operation is near these critical speeds.
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Resonance
• Resonance can be confused with other common problems in machinery.
• Resonance requires some additional testing to be diagnosed.
2
Strong increase in amplitude of the rotation frequency fn at the point of resonance, step-up dependent on the excitation (unbalanced condition) and damping.
ϕ1 = 240° ϕ2 = 60...80°
1.O.
MP MP
1
1.O.
Phase jump by 180°
Resonance Step-up
Grad
rev/min
1 2
rev/min
Amplitude at rotation frequency fn by residual unbalance on rigid rotor.
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Resonance Diagnosing Tests
Run Up or Coast Down Test:
• Performed when the machine is turned on or turned off.
• Series of spectra at different RPM.
• Vibration signals tracking may reveal a resonance.
The use of tachometer is optional and the data collector must support this kind of tests.
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Resonance Diagnosing Tests
Bump Test:
Response – component vibration
t
F/a
5 ms
Double beat
s 1 2
3
t
1 2
3
Decaying function
Excitation – force pulse
Shock component, natural vibration, vertical
t
Frequency response, vertical
Natural frequency, vertical
Frequency response, horizontal
Natural frequency, horizontal
1st mod.
2nd mod.
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©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Journal Bearings
Journal bearings provides a very low friction surface to support and guide a rotor through a cylinder that surrounds the shaft and is filled with a lubricant preventing metal to metal contact.
Clearance problems (rotating mechanical looseness).
Oil whirl
• Oil-film stability problems.
• May cause 0.3-0.5X component in the spectrum.
0,3-0,5X 1X
Radial
High vibration damping due to the oil film:
• High frequencies signals may not be transmitted.
• Displacement sensor and continuous monitoring recommended
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©2011 PRÜFTECHNIK Condition Monitoring – Machinery Fault Diagnosis. Distributed in the US by LUDECA, Inc. • www.ludeca.com
Rolling Element Bearings
• Lifetime exceeded
• Bearing overload
• Incorrect assembly
• Manufacturing error
• Insufficient lubrication
Wear
Wear
1. Wear:
The vibration spectrum has a higher noise level and bearing characteristic frequencies can be identified.
Increased level of shock pulses.
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3
2
1
4
α Angle of contact
D Arc diameter
d Rolling element diameter
Z Number of rolling elements
n Shaft RPM
Rolling Element Bearings
Roller bearing geometry and damage frequencies: 1 - Outer race damage
2 - Inner race damage
3 - Rolling element damage
4 - Cage damage
Dimensions d =77.50 mm D =14.29 mm Z = 10 α = 0
Rollover frequencies BPFO = n / 60 ⋅ 4.0781 = 203.77 Hz BPFI = n / 60 ⋅ 5.9220 = 295.90 Hz 2.fw = n / 60 ⋅ 5.2390 = 261,77 Hz fK = n / 60 ⋅ 0.4079 = 20.38 Hz
Example of rollover frequencies:
Ball bearing SKF 6211
RPM, n = 2998 rev/min
Dw
Dpw
2. Race Damage:
BPFO = ( 1- cos α ) BPFI = ( 1+ cos α ) BSF = ( 1- cos α ) TFT = ( 1- cos α )
Z ⋅ n 2 ⋅ 60 Z ⋅ n 2 ⋅ 60
n 2 ⋅ 60
D ⋅ n d ⋅ 60
d D
2
d D
d D
d D
Ball pass frequency, outer race: Ball pass frequency, inner race: Ball spin frequency: Fundamental train frequency:
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Rolling Element Bearings
Outer race damage frequency BPFO as well as harmonics clearly visible
Outer race damage: (Ball passing frequency, outer range BPFO)
Inner race damage frequency BPFI as well as numerous sidebands at intervals of 1X.
Inner race damage: (Ball passing frequency, inner range BPFI)
BPFO 2⋅ BPFO 3⋅ BPFO 4⋅ BPFO fn
BPFI 2⋅ BPFI 3⋅ BPFI 4⋅ BPFI
Sidebands at intervals of 1X
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Rolling Element Bearings
Rolling element damage: (Ball spin frequency BSF)
FTF and 2nd, 3rd, 4th harmonics
2⋅BSF 4⋅BSF 6⋅BSF 8⋅BSF
Sidebands in intervals of FTF
Rolling elements rollover frequency BSF with harmonics as well as sidebands in intervals of FTF
Cage rotation frequency FTF and harmonics visible
Cage damage: (Fundamental train frequency FTF)
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Rolling Element Bearings
Lubrication Problems:
Lubricant contamination • Race damage
• Defective sealing
• Contaminated lubricant used
Major fluctuation in level of shock pulses
and damage frequencies
Insufficient lubrication → Subsequent
small temperature increase
• Insufficient lubricant
• Underrating
Over-greasing Large temperature
increase after lubrication
• Maintenance error
• Defective grease regulator
• Grease nipple blocked
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Rolling Element Bearings
Incorrect mounting.
Bearing rings out of round, deformed.
• Incorrect installation
• Wrong bearing storage
• Shaft manufacturing error
• Bearing housing overtorqued.
Dirt
Damage frequencies
envelope ↑
Shock pulse ↑ Air gap
Bearing forces on floating bearing.
• Incorrect installation
• Wrong housing calculation
• Manufacturing error in bearing housing
Severe vibration
Bearing temperature increases
Fixed bearing
Floating bearing
Cocked bearing.
• Incorrect installation
Axial 1X, 2X and 3X.
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Blade and Vanes
MP
MP
fBPF
Radial
29
Example characteristic frequency:
3 struts in the intake; x=3.
9 blades; Bn=9.
fBP ⋅ x = N ⋅ Bn ⋅ x
Characteristic frequency = N ⋅ 27
Identify and trend fBP.
An increase in it and/or its harmonics may be a symptom of a problem like blade-diffuser or volute air gap differences.
A blade or vane generates a signal frequency called blade pass frequency, fBP:
fBP = Bn ⋅ N Bn = # of blades or vanes N = rotor speed in rpm
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Aerodynamics and Hydraulic Forces
There are two basic moving fluid problems diagnosed with vibration analysis:
• Turbulence
• Cavitation
MP MP
fBPF 1X 1X
Turbulence: Cavitation:
Random Random
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Belt Drive Faults
MP MP
MP MP Belt transmission a common drive system in industry consisting of:
• Driver Pulley
• Driven Pulley
• Belt
The dynamic relation is: Ø1 ω1 = Ø2 ω2
Ø1
Ø2 ω1
ω2
Belt frequency:
: belt length
lfB
111416,3 φω=
l
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Belt Drive Faults
Pulley Misalignment:
The belt frequency fB and first two (or even three) harmonics are visible in the spectrum.
Radial
2 fB generally dominates the spectrum
Belt Worn:
1X of diver or driven pulley visible and predominant in the axial reading.
fn
1X,2X,3XfB
Offset Angular Twisted
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Belt Drive Faults
Eccentric Pulleys:
The geometric center doesn’t coincide with the rotating center of the pulley.
High 1X of the eccentric pulley visible in the spectrum, predominant in the radial direction.
Easy to confuse with unbalance, but:
• Measurement phase in vertical an horizontal directions may be 0° or 180°.
• The vibration may be higher in the direction of the belts.
Belt Resonance:
If the belt natural frequency coincides with either the driver or driven 1X, this frequency may be visible in the spectrum.
Belt direction
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Gear Faults
Spur Gear:
Planet Gear:
Worm Gear: gear wheel
gear wheel pair
gear train
Driving gear
Driven gear
Gear (wheel) Pinion
Gear
Worm gear
Bevel gear
Bevel gear
Sun gear
Carrier
Planet gear
Ring (cone) Bevel Gear:
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Gear Faults
Starting point of tooth meshing
Pitch point
Working flank
Non working flank
Right flank
Pitch line
Left flank End point of tooth meshing
1 2
3 4 5
6
85
86 87 88
89
Top land
Tip edge
Pitch surface
Root mantel flank
Flank line
Flank profile
Tooth
Tooth space
Root flank
Gear Meshing:
Gear meshing is the contact pattern of the pinion and wheel teeth when transmitting power.
The red dotted line is the contact path where the meshing teeth will be in contact during the rotation.
Gear mesh frequency fZ can be calculated:
Fz = z fn
Where z is the number of teeth of the gear rotating at fn.
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Gear Faults
Incorrect tooth meshing
Wear
MP
MP
z2
z1
fn1
fn2
fz
MP MP X
Detail of X:
fz
fz 2⋅fz 3⋅fz
fz 2⋅fz 3⋅fz 4⋅fz
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Gear Faults
Incorrect tooth shape
MP
MP
fz Detail of X:
X
fz
fz and harmonics
Sidebands
Tooth break-out
MP MP
z2 z1 X
fz
Detail of X:
fn2 fn1
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Gear Faults
Eccentricity, bent shafts
MP MP X
Detail of X:
fz
fz and harmonic sidebands
“Ghost frequencies" or machine frequencies fz fM “Ghost frequency"
Cutting tool
Gearwheel being manufactured
zM
Worm drive part of the gear cutting machine
38
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Electrical Motors
Electromagnetic forces vibrations:
Twice line frequency vibration: 2 ⋅ fL
Bar meshing frequency: fbar = fn ⋅ nbar
Synchronous frequency: fsyn = 2 ⋅ fL / p
Slip Frequency: fslip = fsyn – fn
Pole pass frequency: fp=p ⋅ fslip
fL: line frequency
nbar: number of rotor bars
p: number of poles
• Stator eccentricity
• Eccentric rotor
• Rotor problems
• Loose connections
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Electrical Motors
Stator Eccentricity:
• Loose iron
• Shorted stator laminations
• Soft foot
MP MP
1X
2⋅fL
2X
1X and 2X signals
fL without sidebands
Radial predominant
High resolution should be used when analyzing two poles machines.
Radial
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Electrical Motors
Eccentric Rotor:
• Rotor offset
• Misalignment
• Poor base
MP MP
1X
2⋅fL
2X
Radial
fp
t [ms]
Tslippage
fp, 1X, 2X and 2fL signals.
1X and 2fL with sidebands at fP.
Radial predominant.
High resolution needed.
Modulation of the vibration time signal with the slip frequency fslip Tslip ≈ 2-5 s
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Electrical Motors
Rotor Problems:
1X
Radial
f [Hz]
1. Rotor thermal bow:
• Unbalanced rotor bar current
• Unbalance rotor conditions
• Observable after some operation time
2. Broken or cracked rotor bars: 1X
Radial
2X 3X 4X • 1X and harmonics with sidebands at fP
• High resolution spectrum needed
• Possible beating signal
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Electrical Motors
1X
Radial
f [Hz]
3. Loose rotor bar:
• fbar and 2fbar with 2fL sidebands
• 2fbar can be higher
• 1X and 2X can appear
fn
Radial
2fn 2fL • 2fL excessive signal with sidebands at 1/3 fL
• Electrical phase problem
• Correction must be done immediately
2X fbar 2fbar
Loose connections:
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